The field to which this invention pertains is heat curable coating compositions made from aqueous dispersions of epoxy resins or polyether resins derived therefrom and crosslinking agents, i.e., aminoplast or phenolplast resins or blocked polyisocyanates.
Metal coating compositions based on organic solvent solutions of polyhydroxy polyether resins have been used successfully for a number of years. Such coating compositions have been particularly useful for the interiors of food and beverage containers.
Due to the increasing problems with air pollution, considerable effort is being expended to convert organic solvent containing coating compositions to water based systems, i.e., coating systems based on emulsions and dispersions of film forming resins in water. A serious problem in the use of such aqueous emulsions or dispersions is the inability of the resin to properly wet the metal substrate and to form a continuous coating. In many instances, can coating compositions based on aqueous dispersions of polyhydroxy polyether resins, when spray applied in the manner of conventional solvent based coating compositions, do not form protective continuous coatings. Numerous small voids in the coatings appear as craters or as unwet areas under magnification. Such voids are the cause of objectionable turbidity in beer packaged in aluminum cans, corrosion of steel cans and corrosion of aluminum cans containing carbonated soft drinks.